The invention relates to a gas supply arrangement of a marine vessel and to a method of controlling gas pressure in a gas supply arrangement of a marine vessel.
The usage of gas as energy source in marine vessels is advantageous due to its efficient burning and low emissions. Usually gas is stored in liquefied form because of less space is required for the storage that way.
The propulsion system of LNG (Liquefied Natural Gas) tankers is usually powered by making use of the cargo. Storing of the gas in the tanker is arranged by using heat insulated cargo tanks into which an ullage space section and a liquid phase section are formed. The pressure in the cargo tanks is approximately at atmospheric pressure level and the temperature of the liquefied gas is about minus 163° C. Although the insulation of the cargo tank is extremely good, gradually increasing of the LNG temperature causes formation of so called natural boil-off gas. The natural boil-off gas must be removed in order to avoid extensive increasing of pressure in the cargo tanks. The natural boil-off gas may be utilized in tanker's consumption devices, like in propulsion system. However, the amount of natural boil-off gas is not sufficient for providing all energy required in all circumstances and therefore the vessel must be provided with additional means for acquiring extra gas, so called forced boil-off gas. Additionally the usage of the gas as source of propulsion energy sets requirements for the pressure level and stability of the gas.
For example in a patent publication FR 2722760 there is shown an arrangement in which liquid gas is supplied to a so called forced boiling vaporizer in which the liquid gas vaporizes into gas form, which in turn may be combined with the natural boil-off gas.
EP 1348620 A1 shows a gas supply apparatus in which the natural boil-off gas is led to a compressor, which increase the pressure of the gas prior to feeding it to consumption via a feed line. The capacity of the compressor feeding the natural boil-off gas is controlled by the pressure at the feed line. Additionally, the apparatus also includes a forced boiling vaporizer in which the liquid gas previously pumped to the higher pressure is vaporized. In this arrangement the forced boiling gas portion is combined to the natural boil-off gas after the pressure of the natural boil-off gas has been increased. This arrangement is mentioned to decrease the work required to compress the gas to a given pressure. It has, however, some problems, which are discussed below.
The supply amount of the forced boiling gas is controlled by on-off valve positioned prior to the vaporizer, the control being based on the gas cargo tank pressure. The operation of the pump is also controlled by on-off control based on the pressure of the gas cargo tank. The line between the gas cargo tank and the vaporizer has also a branch for returning a part of pumped liquid gas back to the gas cargo tank. The amount of returned part is controlled based on the pressure of the gas line before the vaporizer. A problem in this arrangement is the returning of the pumped liquid gas back to the cargo tank. That is not desired due to the heating effect of the liquid gas slightly warmed up by the pumping and circulation.
As the consumption of the gas at the outlet side (the feed line) of the vaporizer and compressor varies, the process must be controlled so that the pressure in the gas cargo tank and/or in the feed line will not rise too much and so that required amount of gas may be supplied. When the formation of the natural boil-off gas is not strong enough or consumption is suddenly increasing, the pressure in the ullage space section of the gas cargo tank decreases and the operation of forced boiling vaporizer and liquid gas pump must be started. After operating the pump and the vaporizer a while the pressure will increase to adequate level. Then the pump may be shut down and the valve of the forced boiling vaporizer is also closed. Particularly because the operation of the vaporizer must be precisely controlled, the variation of the pressure in the feed line is inevitably very strong in this kind of a solution. This is even more emphasised by the fact that the vaporizer is typically dimensioned for 100% consumption, which makes the control very rough. Therefore, this kind of a control system is very complicated and also somewhat unstable. Similar arrangement to this is shown in EP 1291576 A2.
An objective of the invention is to provide a gas supply arrangement for a marine vessel, which provides even pressure at the feed line and reliable gas supply for consumption devices of the vessel.